Visible, Near Infrared (NIR), and Short Wave Infrared (SWIR) imagers can encounter up to a nine order of magnitude range in input photon flux density. As such, these imagers require a very wide dynamic range. For example, the illumination level (lux) of a moonless, clear night sky is about 10−3, that of a full moon is about 10−1, while that of full daylight is 1-2×104, and direct sunlight is 1-1.3×105. This very wide dynamic range requirement has historically prevented a single sensor from operating over more than a fraction of the total dynamic range. For example, a sensor optimized for a moonless or even a moonlit night is essentially blinded or saturated in daylight.
Conventional readout circuit amplifier types include a high gain amplifier type known as the charge transimpedance amplifier (CTIA), sometimes referred to as a reset integrator, while a lower gain amplifier type is known as a source follower per detector (SFD). Reference in this regard can be made to, as examples U.S. Pat. No. 4,445,117, “Transistorized Focal Plane Having Floating Gate Output Nodes”, by Steven D. Gaalema et al.; U.S. Pat. No. Re. 34,908, “3-Transistor Source Follower-Per-Detector Unit Cell for 2-Dimensional Focal Plane Arrays”, by Richard H. Wyles et al.; and U.S. Pat. No. 4,786,831, “Integrating Capacitively Coupled Transimpedance Amplifier”, by Arthur L. Morse et al. The disclosures of these now commonly assigned U.S. patents are incorporated by reference herein their entireties.
The wide dynamic range imager requirement has been approached in a conventional readout circuit by the use of a single circuit type, such as the CTIA, that is configured to have two or more distinct gains and variable integration times. However, the use of multiple gains in such circuits becomes progressively more difficult as the readout circuit unit cell size is made smaller, as is typically the case when the number of discrete imager pixels is increased.
Other conventional approaches to accommodating the wide dynamic range requirement include the use of a current mirror, or the use of a resistor load gate modulation circuit. While these two types of circuits are capable of operation over a wide dynamic range, they suffer from poor uniformity and are difficult to calibrate. These circuits also suffer from poor frequency response at very low light levels.
Prior to this invention the problems presented by the inherent wide dynamic range requirement of modern radiation sensors arrays have not been adequately addressed.